docs: def add_cell_code(self, cell_str, pos=None):
          """
          Add Python cell
          :param cell_str: cell content
          :return:
          """
  
          cell_str = cell_str.strip()
  
          logging.debug("add_cell_code: {}".format(cell_str))
          cell = nbf.v4.new_code_cell(cell_str)
  
          if pos is None:
              self.nb['cells'].append(cell)
          else:
              self.nb['cells'].insert(pos, cell)